This invention relates to digital display systems.
A digital display is normally formed as an array of picture elements whose intensities can be individually controlled. Examples are digitally controlled raster scanned CRTs and digital plotters. Early displays used only two values for the intensity of the picture elements--they were either illuminated or not illuminated--but more recent displays have allowed a greater range of intensities, either as a grey-scale monochrome display or as part of a color display.
It has been proposed to exploit the possibilities offered by a range of intensities to overcome one of the most disturbing features of displays with only two intensities, namely the jagged edges possessed by lines at an oblique angle. Picture elements in the neighborhood of an edge are given a graded intensity in a way that causes the edge to appear smooth even if it is at an oblique angle. In my U.S. patent application Ser. No. 488,368, I describe one such system in which the intensity of a picture element in the neighborhood of an edge is determined from its perpendicular distance from the edge. The system is capable of displaying both straight lines and circles; for circles the perpendicular distances for all points in the neighborhood of the circle are precalculated for each radius allowed and then stored in a look-up table. This method, while satisfactory for small radii, leads to inconveniently large look-up tables for large radii.
This invention provides a method of displaying an arc of a circle on a display device which produces an image as an array of picture elements arranged in lines, each picture element being capable of being displayed with a selected one from more than two possible intensities, the method comprising:
(A) for each of the said lines of picture elements that crosses the arc conducting a scan which comprises PA0 (B) controlling the display device so that picture elements included in the said scans are displayed with the indicated intensity. PA0 (a) a first and a second picture-element address register, the first indicating a position for the picture element in the one of the said lines in which it lies and the second indicating a position for that line; PA0 (b) sequencing means for conducting a sequence of scans, the sequencing means in each such scan repeatedly stepping the contents of the first address register from the value corresponding to one picture element to that corresponding to another and at the conclusion of each such scan stepping at least the contents of the second address register from the value corresponding to one line to that corresponding to another; PA0 (c) means for holding an indication of the closest distance from the picture element whose position is indicated by the address registers to an ideal circle characterising the arc; PA0 (d) means for incrementing or decrementing the contents of the means for holding the closest-distance indication to correspond to a step in each of the said scans; PA0 (e) means responsive at least to the output of the means for holding the closest-distance indication to output an intensity indication for the element whose position is indicated by the address registers; and PA0 (f) means responsive to the intensity indications for issuing a drive signal to control a digital display device to display corresponding picture elements with the indicated intensity.
(a) for an initial picture element of the scan line deriving an indication of the closest distance from a sampling point corresponding to the element to an ideal circle characterising the line, PA1 (b) for each subsequent picture element of the scan deriving an indication of the closest distance from a sampling point corresponding to the element to the said ideal circle by incrementing or decrementing the corresponding indication of the preceding picture element, and PA1 (c) for each picture element of the scan deriving from at least the closest-distance indication for that element an indication of an intensity for the picture element; and
This invention also provides apparatus for controlling a display device to display an arc of a circle, the display device producing an image as an array of picture elements arranged in lines, each picture element being capable of being displayed with a selected one from more than two possible alternatives, the apparatus comprising: